1. Field of the Invention
The present invention relates to a magnetic field generator of a magneto-optical recording apparatus in which information is magneto-optically recorded and erased by irradiating a laser beam to a magneto-optical recording medium from one side and applying a magnetic field thereto from the opposite side.
2. Related Background Art
When information is recorded and erased in that type of magneto-optical recording apparatus, an optical head is used to irradiate a laser beam emitted from a semiconductor laser to a magneto-optical recording medium in the form of a disc, for example, and the external magnetic field is also applied to the magneto-optical recording medium vertically at the same position as that being irradiated. The magnetic field generator is of the floating type as shown in FIG. 1, for example, and is disposed above the disc-like magneto-optical recording medium in opposite relation to the underlying optical head (not shown). A slider 81 for producing a floating force is made of non-magnetic material and has two slide surfaces 82, 83 on both sides of a center groove. A core 84 for generating the vertical magnetic field is buried in an air outflow portion (i.e., a portion from which airflow in the direction of travel of the magneto-optical recording medium is discharged) of one slide surface. A winding window 85 is defined by forming a cutout in the rear end portion of the slider 81, and a coil 86 is wound around the core 84 through the winding window 85. As a result, a magnetic path is made open outwardly at the end of the core 84 with magnetic poles l.sub.1 and l.sub.2 arranged in the surface of the slider 81 facing the magneto-optical recording medium (denoted by reference numeral 93 in FIG. 2). Then, as well known, the coil is supplied with a signal voltage to apply the vertical magnetic field to a recording layer (denoted by 94 in FIG. 2) of the magneto-optical recording medium.
On the other hand, as shown in FIG. 2, a light beam 91 from a semiconductor laser is focused through an object lens 92 to a focal point S on the recording layer 92. At this time, the core 84 is positioned on the opposite side of the magneto-optical recording medium 93 in facing relation to the object lens 92 so that the vertical magnetic field is applied from the magnetic pole l.sub.1 to the recording layer 94. Usually, the focal point S is moved on the order of about .+-.250 .mu.m in the radial direction of the magneto-optical recording medium (for so-called tracking) by movement of the object lens 92 in tracking control without moving the optical head, and the magnetic pole l.sub.1 of the core 84 is set such that its effective vertical magnetic field has the width size to substantially cover a movable scope of the focal point.
Further, for the purpose of efficient generation of the magnetic field, the core 84 is in the form of a horseshoe or U to make the magnetic path open outwardly. As seen from FIG. 1, the magnetic pole l.sub.2 for generating the magnetic field of opposite magnetism to the magnetic pole l.sub.1 is also exposed to the slide surface 83. To secure a sufficient area of the winding window 85, the spacing between the magnetic poles l.sub.1 and l.sub.2 is relatively wide, on the order of several hundreds of microns.
Considering an alignment error between the core 84 and the object lens 92 as well, however, the core 84 is required to have a width of about .+-.300 .mu.m in the radial direction of the magneto-optical disc (i.e., in a direction orthogonal to the direction of a track thereof) and a length of about .+-.100 .mu.m in the direction of a track. On the other hand, the relationship between a scope of the effective vertical magnetic field generated by the core and maximum modulation frequency of the magnetic head at that time is in inverse proportion as plotted in FIG. 3. Stated otherwise, since the effective magnetic field scope of the magnetic head corresponds to a sectional area of the core magnetic pole, the effective magnetic field scope is substantially in match with the sectional area of the magnetic pole l.sub.1 of 0.6 mm.times.0.2 mm=0.12 mm.sup.2 and, therefore, the maximum modulation frequency is about 2.5 MHz.
Meanwhile, requirements for characteristics of the magneto-optic recording apparatus are increased year by year to be adapted for speed-up. It is naturally desired to set the higher maximum modulation frequency and, as a result, the foregoing 2.5 MHz is insufficient. Also, since the spacing between the two magnetic poles l.sub.1 and l.sub.2 is wide, as large as about 100 .mu.m, the magnetic resistance for forming the magnetic path between those two magnetic poles is too high to generate the vertical magnetic field efficiently and sufficiently.
It has, therefore, been proposed to reduce the size of a core in a magnetic head and arrange the small-size core plural in number. More specifically, as shown in FIG. 4, the proposed construction includes a pair of small-size cores 111, 113 around which coils 112, 114 are respectively wound to be driven independently of each other. As compared with the aforementioned case of using a single core, the required scope of the effective vertical magnetic field can be reduced by almost half and the maximum modulation frequency can be increased almost twice in the case of using the two cores 111 and 113. However, this case also has other problems explained below.
Specifically, in a graph on the lower side of FIG. 4, a solid line 117 represents the intensity of the magnetic field versus a horizontal position x spaced several tens of microns from the lower end face of the core 111 when a winding 112 coiled around the core 111 is supplied with an electric current to generate the upward magnetic field +H (indicated by an arrow 115). As will be seen from FIG. 2, as the position x is further apart away from the core end face, the magnetic field generated becomes weak to such an extent that it is smaller at a middle position between the core 111 and the adjacent core 113 than the vertical magnetic field of 200 [Oe] necessary for recording of information to a recording medium. Accordingly, to obtain the required magnetic field at the middle point between the two cores 111 and 113, the electric current supplied to the coil must be increased, which is disadvantageous in driving at high frequency. This will offset the benefit of increasing the modulation frequency due to a reduction in size of the cores.
Furthermore, when carrying out multi-beam recording to meet the requirement of speed-up of the magneto-optical recording apparatus, the modulation magnetic fields to be modulated in accordance with recording information are required to be applied independently of each other in irradiating a plurality of light beams to a magneto-optical recording medium for formation of spots at predetermined positions. Here, a coil 114 wound around the core 113 is supplied with an electric current in such a manner as to generate the magnetic field in an opposite direction to that generated by the coil 112 wound around the core At this time, the core 113 generates the downward magnetic field -H (indicated by an arrow 116) and, similarly to the above case, the intensity of the magnetic field versus the horizontal position x is represented by a solid line 118. The vertical magnetic field actually applied to the recording medium from the magnetic fields 117, 118 respectively generated by the two cores 111, 113 is represented by a dotted line 119. As a result, even at central positions x.sub.111, x.sub.113 of the cores 111, 113 with respect to the recording medium, the vertical magnetic field higher than .+-.200 [Oe] necessary for recording cannot be obtained so that the multi-beam recording fails to perform. Spacing the two cores farther away from each other so as to avoid an influence of mutual interference therebetween, however, means larger separation between the plural light beams in a single optical head. This is difficult and infeasible to carry out from the practical standpoint.